Ballasts are rated for wattage. Yes they are different pin spacings. T8 and T12 are medium bipin and T5 is mini bipin. IF (and that's a big IF) the socket was changed in the fixture to a mini bipin a normal output T5 bulb would work.....but that's alot of work for nothing gained.

Some ballasts will do either or both. it will say on the ballast itself. It gets tricky trying to DIY with flourescent tubes. They require a specific capacitance, that the metal base and reflector provides, to fire.